Radon is a radioactive gas that occurs naturally in soil gas present underground. Soil gas can flow into a building through openings where the floor and walls contact the soil. Prolonged human exposure to elevated concentrations of radon and radon decay products has been recognized as a health hazard. Radon is particularly a problem in slab-on-grade and basement construction housing where a thermal stack effect, air exhausting appliances, or similar factors cause a pressure gradient across the slab so that air pressure indoors is less than air pressure in the surrounding soil. This results in a phenomenon known as depressurization wherein radon is drawn from the soil through cracks and openings into the building.
The amount of radon that enters the building structure is a function of how much radon gas or radon parent compounds are present in the sub-slab soil, the permeability of the soil, the cracks and openings between the soil and the building interior, and the forces that drive the radon-containing gas along the pathways into the structure. Radon level mitigation techniques include blocking entry using barrier methods, reducing radon entry driving forces, and diverting radon from entering using sub-slab ventilation and depressurization.
Sub-slab depressurization (SSD) is perhaps the most successful and widely used technique to date for reducing radon concentration in slab-on-grade and basement houses with sub-slab communication. Installation of an SSD system involves the insertion of vent pipes through penetrations in the slab to access suction points in the aggregate or permeable soil beneath. Fans or air pumps are used to suck the radon-containing soil gas from under the slab through the pipes, releasing it outdoors. This sub-slab suction method works by creating a negative (relative to building interior) pressure field under the slab which prevents the gas from entering the building. A primary sub-slab suction point is normally located centrally of the area to be mitigated, though additional suction point locations may be necessary if subslab air flow communication (i.e., pressure field extension) is insufficient.
In a typical method of remedial radon reduction using an SSD approach, a hole is drilled through the slab and into the underlying soil at the chosen suction point location and an open or aggregate filled suction pit is established below the slab in communication with the hole penetration. The layouts of utility pipes and sub-slab conduits are noted beforehand to avoid drilling through them. A vent pipe is then inserted through the slab into the pit to establish a vertical drop or riser of suction pipe from the pit to a point, such as in the attic, located outside the conditioned space of the building. An exhaust fan is then connected to the upper end of the pipe to provide suction for drawing the gas up the stack and exhausting the same externally of the building. The run of riser pipe is continuously extended from the sub-slab pit location to the attic, with bends and elbows selected to minimize pressure drops in the piping. The vent pipe fan and all pressurized portions of the pipe located outside the conditioned space are sloped to drain condensation back to the sub-slab soil and prevent accumulation of water in the pipe or fan housing. The annular region between the vent pipe and the slab penetration hole is provided with an airtight seal.
Where sub-slab permeability is inadequate to allow the required pressure field extension, additional sub-slab suction points may be located and vent pipe upper ends joined by an attic manifold for connection to a common exhaust fan. The fan is located for release of exhausted air outside the building, preferably above the roof--away from operable roof openings, air intakes or open windows. Coverage may be increased if a larger capacity fan and/or greater diameter vent pipe is used. The SSD system is normally used in conjunction with steps taken to seal entry points such as cracks, joints, utility openings, etc.
A certain amount of sealing is recommended for sub-slab suction mechanisms to work effectively. Conventional construction often situates HVAC (heating, air-conditioning and ventilation) units at central locations and connects refrigerant lines from such units to the outside through open-ended hollow ductwork, chases or similar conduits (hereafter "chases") which traverse a path from an interior point above the slab, through the slab, and then along a sub-slab path generally horizontally to an external exit point. Similar chases may be utilized to carry electrical wiring, telephone or other utility conduits. To avoid radon entry through such chases, the conventional approach to radon remediation recommends either that the ends be fully sealed to close the gap between the chase inside diameter and the outside diameters of the conduits located therein or preferably, if feasible, that any sub-slab ductwork be sealed off completely and replaced with new above-slab ductwork. For new construction, above-slab ductwork is suggested. In new construction, it is also recommended too that the slab supporting sub-soil have good gas communication and to consider preplacing stub-up pipes at the time of pouring of a monolithic slab to leave capped connections for use in subsequent attachment to vertical risers installed later, should a sub-slab ventilation system become advisable.